Ivory poachers are increasing using poisons to kill African elephants. And it's having a deadly knock-on effect for critically endangered and rare species of vultures.

A recent study suggests that poachers are using poison for two reasons. First, they want to kill the elephants for their valuable ivory tusks, which are mostly being exported to China and elsewhere in East Asia.

Second, vultures circling in the sky over dead elephants can alert authorities that poachers are active in an area. By poisoning the elephant or its carcass, the poachers can eliminate this natural 'alarm call' and thereby remain hidden from park guards.

Circling vultures... help to reveal poachers.

Lead author Dancy Ogada from The Peregrine Fund, an international organization devoted to conserving birds of prey, said poisoning had "exploded in magnitude" as a killer of elephants and rare vultures. Poisoning is now "the biggest cause of vulture mortality" in Africa, he said.

In Africa, many poisons and pesticides are weakly regulated, making it easy for poachers to get their hands on them.

Largely as a result, in recent months six species of African vultures have been listed as Critically Endangered or Endangered because of their increasingly catastrophic mortality. Africa has the world's richest assemblage of vultures, with 11 of the world's 23 species found on the continent.

Many vulture species raise just a single chick at a time, making them highly vulnerable to population crashes

In the past, poachers in Africa have used modern technologies such as high-powered rifles and cable snares to kill elephants. Now, a variety of poisons are being added to their arsenal, with increasingly deadly effects on Africa's diverse assemblage of vulture species.

Poisons are also being used to kill African rhinoceros. According to the authors, at least six rhinoceros and 181 elephants are known to have been poisoned in Botswana, the Democratic Republic of Congo, Mozambique, Namibia, South Africa, Tanzania, Zambia, and Zimbabwe from 2012 to 2014.

The authors of the recent paper argue that authorities need to do much more to regulate access to the readily available poisons and pesticides that poachers are using.

The Peregrine Fund is soliciting public support to aid its efforts to reduce the increasingly deadly impact of poisons on endangered vultures worldwide.